1. Field of the Invention
This invention relates to a photosensitive element for flexographic printing and is applicable to a plate-making process by which a digitized image is directly drawn by an infrared laser without using a phototool such as a negative film.
2. Description of the Related Art
A conventional photosensitive element for flexographic printing is generally composed of a polyester film or the like as a support and a photosensitive resin layer placed thereon comprising a thermoplastic elastomer, at least one monomer and at least one radiation-sensitive initiator. In the procedure for making a flexographic printing plate from such a photosensitive element for flexographic printing, the whole surface of the element is first exposed to an ultraviolet light through the support (back exposure) to prepare a thin, uniform cured layer. Subsequently, the photosensitive resin layer surface is subjected to image exposure (relief exposure) and the unexposed portion is removed by washing with a developing solvent to obtain the desired image, namely relief image, from which a printing plate is obtained. In this case, a thin film called slip layer or protective layer is often provided on the photosensitive resin layer for the purpose of smoothing contact between the photosensitive resin layer and a phototool such as a negative film.
There is also known a photosensitive element for flexographic printing on which the digitized image information can be directly drawn without using a phototool such as a negative film and a plate-making process using the photosensitive element. In the procedure thereof, the "non-infrared ray"-shielding thin layer provided on a photosensitive resin layer is ablated with an infrared laser based on the digital information processed by a computer, to obtain the desired image. After drawing an image on the photosensitive resin layer, a conventional plate-making process can be applied to the photosensitive element in which the photosensitive resin layer has an image drawn thereon.
That is to say, using the existing exposure apparatus, the photosensitive element is subjected to back exposure from the support side and then to relief exposure from the side of the image drawn with an infrared laser, and thereafter, subjected to development step, whereby a flexographic printing plate is formed. As compared with the conventional method using a phototool such as a negative film, when an amendment to the image has become necessary, the above plate-making process does not require the preparation of a new phototool such as a negative film and the digitized image data can be amended on a computer, so that the above plate-making process has such an advantage that time and labor can be saved. Moreover, as compared with the conventional phototool such as a negative film, the photosensitive element is advantageous in dimensional stability, and this results in an improvement in reproducibility of relief image which brings about an improvement in printing quality.
As to the non-infrared ray-shielding layer which can be ablated with an infrared laser, JP-A-8-305,030 describes that as the binder polymer therefor, there is used a polymer substantially incompatible with at least one low molecular weight material of the photosensitive resin layer. As examples of the binder polymer to be used for the purpose of realizing such characteristics, said publication mentions polyamide, polyvinyl alcohol, polyvinyl alcohol/polyethylene glycol graft copolymer, amphoteric interpolymer, alkyl cellulose, hydroxyalkyl cellulose, nitrocellulose, copolymer of ethylene and vinyl acetate, cellulose acetate butyrate, polybutyral, cyclized rubber, copolymer of styrene and acrylic acid, polyvinyl pyrrolidone and copolymer of polyvinyl pyrrolidone and vinyl acetate; combinations of polymers selected from a group of these polymers; and the like.
However, a non-infrared ray-shielding layer prepared from these polymers often causes an inconvenience resulting from its poor compatibility with the low molecular weight material of the photosensitive resin layer. For example, it is unavoidable that the combination of the non-infrared ray-shielding layer with the photosensitive resin layer becomes such a combination that the affinity between the two is small. Therefore, the adhesion between the two is small, and when the cover sheet is peeled before the drawing of an image with an infrared laser, such a phenomenon occurs often that the non-infrared ray-shielding layer is partially peeled from the photosensitive resin layer and consequently the partially peeled non-infrared ray-shielding layer is removed as attached to the cover sheet. When a plate is set to a drum of a laser image drawing machine to draw an image, the plate is generally fixed to the drum with a tape on the plate. However, when the adhesion between the non-infrared ray-shielding layer and the photosensitive layer is not sufficient, this portion is peeled during the rotation of the drum to cause the detachment of the plate, resulting in serious damage of both the plate and the laser image drawing machine. In order to prevent such an inconvenience, after the non-infrared ray-shielding layer around the plate is peeled from the photosensitive resin layer in advance, the plate is fixed at that portion with a tape. However, this step remarkably reduces the productivity of the plate-making process.
Furthermore, there is a difference in dissolvability in a developing solvent between the photosensitive resin layer and the non-infrared ray-shielding layer prepared using the above-mentioned polymers. Therefore, there is a problem that a solvent suitable inherently for the photosensitive resin layer occupying a major portion of the photosensitive element for flexographic printing cannot be used as a developer.
For example, in the photosensitive resin layer of a typical photosensitive element for flexographic printing, a thermoplastic elastomer composed of a polymer of a styrene derivative and a conjugated diene monomer is used as its binder polymer.
As a developer, chlorine-free solvents have been used recently due to concerns of protecting the environment, which are a mixture of a petroleum hydrocarbon or ester solvent with an alcohol. However, when these chlorine-free solvents are used, there may be a problem in some cases due to the difference in dissolvability in the solvent between the photosensitive resin layer and the non-infrared ray-shielding layer which can be ablated with an infrared laser.
For example, when a photosensitive resin layer in which a thermoplastic elastomer is used as a binder polymer is combined with a non-infrared ray-shielding layer in which a polyamide is used as a binder polymer, the use of 3-methoxybutyl acetate as a developing solvent results in an insufficient removal of the non-infrared ray-shielding layer by washing. In addition, even when combined with a non-infrared ray-shielding layer in which cellulose acetate butyrate is used as a binder polymer, the use of a combination of a petroleum hydrocarbon with an alcohol as a developing solvent similarly results in an insufficient dissolvability and accordingly the problem that the depth of relief after the development is not uniform. Moreover, the non-infrared ray-shielding layer which has not been completely dissolved in the developing solvent becomes swollen with the solvent and floats in the developing tank. This swollen layer floating in the tank causes a blocking of the piping for circulating the liquid, or often causes serious problems in the plate-making step such as re-attachment of the floating material to the plate and the like.
An object of this invention is to provide a photosensitive element for flexographic printing applicable to the plate-making process in which a digitized image is drawn with an infrared laser directly without using a phototool such as a negative film, wherein a novel non-infrared ray-shielding layer which can be ablated with an infrared laser is used to make a good adhesion of the non-infrared ray-shielding layer to the photosensitive resin layer, thereby solving such problems that the peeling of the non-infrared ray-shielding layer can cause during the peeling of a cover sheet, and the like.
Another object of the invention is to provide a photosensitive element for flexographic printing which enables the selection of a developing solvent in a broad range.